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In Sex, Brain Studies Show, 'la Différence' Still Holds

For almost a decade, researchers at Pfizer struggled to show that Viagra, the male impotence drug, could enhance sexual function in women.

Last month, they gave up.

Countless tests on thousands of women made it clear that the little blue pill, though able to stir arousal, did not always evoke sexual desire.

Viagra's failure underscored the obvious: when it comes to sexuality, men and women to some extent are differently tuned. For men, arousal and desire are often intertwined, while for women, the two are frequently distinct.

Scientists have recently begun to map out how this difference shows up in the brain.

For example, male arousal, studies find, is strongly visual, and when men engage in sexual activity or even anticipate it, brain structures once thought to have little connection to sex spring into action. The same brain regions, however, remain relatively quiet when women are aroused.

At the core of the sexual divide, some researchers say, is the amygdala, an almond-shaped nugget embedded in the limbic system, the brain's seat of emotions. Once thought to be involved exclusively with emotions like fear and anger, the amygdala is now believed to be more complicated.

In one recent study, a team of researchers at Emory University had 28 men and women look at erotic photographs while an M.R.I. took snapshots of their brains. A pattern immediately emerged. The photographs set off a frenzy of brain activity, particularly in the amygdalae of men. Yet the two groups reported equal arousal most of the time.

''This definitely emphasizes that up until recently the amygdala has been overlooked,'' said Dr. Stephan Hamann, a professor of psychology and the lead author of the study, which was published online by Nature Neuroscience last week and is scheduled to appear in the journal in April. ''Often, the amygdala is involved in the anticipation of positive emotions, which this parallels nicely with.''

Much of the insight into the amygdala's role in anticipating sex, and possibly other pleasant emotions, stems from research on animals.

In a study in 1989, scientists trained caged male rats to gain access to females by pushing a lever. The researchers then destroyed part of the amygdala. The rats lost interest in pressing the lever. Yet despite this lack of motivation, they had no problem engaging in sexual intercourse when the females were placed in their cages.

Other studies have gone further. In a study published last year, researchers in the Netherlands recorded brain activity in men as their female partners brought them to orgasm.

The amygdala, the scientists found, showed decreased activity during climax. Other studies have suggested that a larger amygdala may lead to a more robust sex drive.

Dr. Hamann pointed out that the amygdala is known to have intricate connections to primates' visual systems. One reason for the powerful response to visual stimuli in men, he said, could be cultural. Men tend to be inundated with sexual imagery and, possibly, are more likely to seek it out.

Evolution may also have a role. Some experts argue that, over time, men naturally became more dependent than women on sight in selecting a mate.

''For millions of years, men have had to size up a woman's reproductive capacity by looking for signs of youth and health that would enable them to carry a healthy baby,'' said Dr. Helen Fisher, an anthropologist at the Center for Human Evolutionary Studies at Rutgers, and the author of a recently published book, ''Why We Love: The Nature and Chemistry of Romantic Love.''

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Dr. Fisher has studied the brains of people in the early stages of romance. For a man, she found, pictures of a new partner light up parts of the brain involved in visual processing and arousal. But women, she noticed, show more activity in areas linked to reward, emotion and attention.

''Men, despite what most people think, fall in love faster than women do, probably because they're so visual,'' Dr. Fisher said.

Women, on the other hand, may be more attuned to the signs that a man will make a good father or provider, she added, though some other researchers find this assertion dubious. A woman's choice of a mate, Dr. Fisher argues, could involve an interplay among a number of factors, including, some experts now theorize, a man's odor.

Several years ago, Swiss scientists discovered that women could sniff out genetic differences in potential mates. When women were asked to smell T-shirts that different men had worn, they often ranked more favorably the shirts that belonged to men with dissimilar genes for major histocompatibility complex, a group of proteins involved in immunity to disease. The odors a woman preferred also tended to remind her of past and current partners.

Seeking out different immune-system genes might be a way to prevent inbreeding or to arm offspring with a more versatile immune system, said Dr. Rachel S. Herz, a psychologist at Brown who in a study in 2002 found that women ranked body odor above almost every other factor in attraction, except ''pleasantness.''

''For women, the costs of pregnancy like time and energy are pretty high,'' Dr. Herz said. ''So to balance those costs, you want to make sure the child is going to live. And what is indicative of how healthy you are is your immune system, which is manifested in your smell.''

Yet to play down the role of visual stimulation for women would be unwise, researchers say, and female admirers of Brad Pitt or George Clooney would probably agree. In research last year at Northwestern, Dr. Meredith Chivers, demonstrated that women could sometimes have more powerful responses to visual stimuli than men, although in different ways.

In her study, which ignited a small firestorm, Dr. Chivers used a device to measure genital arousal in subjects as they looked at pornography. Heterosexual men, she found, were aroused by footage of men and women having sex. Gay men reacted to two men having sex. Women, regardless of sexual orientation, responded to everything.

In some cases, she said, women reported no sexual arousal, though the device said otherwise.

''One of the fascinating things was that the female responses to sexual images were fast and automatic,'' said Dr. Chivers, who is now at the Center for Addiction and Mental Health in Toronto. ''The fact that they were not always aware speaks to there being other factors involved like emotion and psychological influences.''

What is apparently a disconnection between female arousal and desire, researchers note, squares nicely with the Pfizer findings on Viagra.

Still, other experts point out that it is impossible to define neatly or predict how men and women will respond. People's perceptions are colored by their personal experiences, said Dr. Leonore Tiefer, a sex therapist who is a clinical associate professor of psychiatry at the New York University medical school.

''Differences between genders are boring,'' Dr. Tiefer said. ''The big differences are within the sexes, between individuals. It is not the case that every person pays attention to the same thing.

''It's like everything else in life -- eating, dancing, traveling. The whole experience is shaped by your history and by what you're paying attention to.''

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A version of this article appears in print on March 16, 2004, on Page F00005 of the National edition with the headline: In Sex, Brain Studies Show, 'la Différence' Still Holds. Order Reprints|Today's Paper|Subscribe